254 Profs. J. Dewar and J. A. Fleming. Dielectric Constants 



Table III. — Comparison of the Determinations of certain Dieleotric 



Constants made by different Methods at the Temperature of 

 Liquid Air. 



By galvanometer By Nernst's bridge 



and switch method. method with telephone. 



Frequency = 120. Frequency = 320. 



Substance. Dielectric constant. Dielectric constant. 



Ethylic alcohol 311 313 



Amyl alcohol.. 2*14 258 



Ethylic ether 2 31 2 29 



5 per cent, solution (aqueous) 



of Potassic Hydrate 123*0 7'12 



5 per cent, solution (aqueous) 



of Rubidic Hydrate 816 3 55 



The results collected in the above Table III, show that the two 

 methods give practically identical values for the two alcohols and 

 the ether, but very different value for the two frozen dilute hydrates 



An examination was then made of several other substances, and 

 for this purpose another condenser was constructed, which consisted 

 of a platinum crucible about 4 cm. in diameter and 5 cm. high. 

 This crucible was fitted with an ebonite lid, through which passed a 

 glass test-tube, in the interior of which was placed our platinum 

 thermometer. Bound the outside of the test-tube, platinum wire 

 was closely wound, so as to form the opposed surface of a condenser 

 in relation to the platinum crucible as the other surface. This 

 platinum condenser could then be filled with any electrolyte or 

 organic liquid and frozen in liquid air. 



Owing to the very small actual capacity of this last experimental 

 condenser, and especially that of the variable part of it in comparison 

 with the capacity of the leads and connections, no very great accuracy 

 of measurement was looked for or attained. The results, however, 

 were sufficient to check the general [accuracy of the experiment with 

 similar substances by the galvanometer method. This platinum 

 condenser was calibrated and used with the rTernst bridge, exactly 

 as in the previous experiment. 



With the experimental condenser empty the change in capacity of 

 the variable sliding condenser in the bridge arm was 1'50 on chang- 

 ing over the position of the experimental condenser. Hence s = 1'50. 



When filled with ethylic alcohol (D = 25*8) the change of capacity 

 of the sliding condenser was 6*20. We have, therefore, S = 6*20, D ( > 

 = 25-8, s= 1-50. 



Therefore Po ~ T = 248 = 5'27. 



S — s 470 



The experimental condenser was then filled with some liquid, 

 either at ordinary temperature or frozen at a low temperature, and 



